1. Field of the Invention
The present invention relates to a power steering apparatus suitable for use in vehicles, and more particularly, to a power steering apparatus having a drooping mechanism capable of decreasing the flow rate of operating fluid supplied to a power assist gear unit thereof during high rotational speeds of an engine-driven pump.
2. Discussion of the Prior Art
For energy saving, a conventional hydraulic power steering apparatus is provided with a drooping mechanism, which operates to reduce the flow rate of operating fluid discharged from an engine-driven pump to a power-assist gear unit thereof when the rotational speed of the pump exceeds a predetermined value (i.e., in the high speed traveling of the vehicle). The provision of the drooping mechanism is because it is general that the power assist is hardly required during the high speed traveling and that the force to keep a steering wheel of the vehicle to its neutral position is rather required for the stability in handling, for which purpose the power assist force has to be reduced.
FIG. 1 shows the fluid circuit diagram of the aforementioned conventional power steering apparatus with such a drooping mechanism. The power steering apparatus comprises an engine-driven pump 50, a power assist gear unit 90 operated by means of operating fluid discharged from the pump 50, a supply passage 80 supplying the operating fluid from the pump 50 to the gear unit 90, and a flow control valve 70 for maintaining the flow rate of the fluid toward the gear unit 90 almost constant. The drooping mechanism comprises a control orifice 40 on the supply passage 80 and a variable throttle valve 30 arranged at the downstream of the control orifice 40 to operate in response to the pressure difference across the control orifice 40. The variable throttle valve 30 constitutes a flow control device together with the flow control valve 70.
FIG. 2 shows the concrete construction of the flow control device incorporating the drooping mechanism therein. The flow control valve 70 primarily comprises a spool 710 and a spring 720 and is formed with an oil inlet chamber 410 to which the operating fluid from the pump 50 is admitted. The control orifice 40 is provided at the downstream of the oil inlet chamber 410, and a sub-spool 310 is provided in turn at the downstream of the control orifice 40. A communication passage 420 is formed to lead the operating fluid from the oil inlet chamber 410 to the shoulder portion of the sub-spool 310. A metering orifice 330 is further provided at the downstream of the sub-spool 310. When moved against the force of a spring 320 in response to the pressure difference across the control orifice 40, the sub-spool 310 reduces the opening area of the metering orifice 330.
In the flow control device of the construction above, the operating fluid discharged from the pump 50 is supplied to the oil inlet chamber 410. The operating fluid, after being controlled by the control orifice 40, passes through a spool head chamber 430 and a through hole 340 of the sub-spool 310 and then, is controlled by the metering orifice 330 to a predetermined flow rate so as to be delivered from a delivery port 810 to the power assist gear unit 90. When the rotational speed of the pump 50 rises thereby to increase the discharge volume from the pump 50, the difference between the pressures at the upstream and downstream of the control orifice 40 increases. This causes the sub-spool 310 to move against the force of the spring 320, and the opening area of the metering orifice 330 is reduced, whereby the flow rate of the operating fluid which supplied to the power assist gear unit 90 through the metering orifice 330 is decreased: namely, a so-called drooping operation is carried out. This drooping operation works for the stability of the vehicle in a high-speed traveling.
However, the pressure within the flow control device including the flow control valve 70 is raised during the drooping operation being carried out. More specifically, although increases in the rotational speed and hence, the discharge volume of the pump 50 causes the pressure difference across the metering orifice 330 to increase, changes in the pressure difference act on the sub-spool 310 to reduce the opening area of the metering orifice 330. As a consequence, the flow resistance of the metering orifice 330 is increased, whereby the pressure in the oil inlet chamber 410 when the opening area of the metering orifice is not reduced becomes higher than when the opening area is reduced. The pressure increase in the oil inlet chamber 410 disadvantageously results in imposing a larger load on the pump 50, so that during the drooping operation, more engine power is consumed at the sacrifice of the stability of the vehicle in the high-speed traveling.
Another power steering apparatus has been known which, as described in Japanese examined, published patent application No. 54-5571, is provided with a flow control device capable of relieving the load imposed on an engine-driven pump during high speed traveling of the vehicle. In this known apparatus, the pressure of the operating fluid is lowered by draining the fluid within a spring chamber of a flow control valve during high speed traveling of the vehicle. However, this known apparatus requires a vehicle-speed sensing means, an electric control device, a solenoid-operated valve and the like and therefore, results in high cost. Power steering apparatus of the similar type have also been known by U.S. Pat. Nos. 4,609,331 and 4,714,413 to James J. Duffy.